The present invention relates generally to electrical panels and, more particularly, to a power supply or input interlock assembly constructed to electrically isolate different inputs connected to an electrical panel.
Electrical panels, breaker boxes, or load centers frequently include a main contactor, switch, or breaker, which electrically isolates a series of load breakers from a utility power input. Occasionally, such load panels are configured to receive another input power source, such as from a generator, to provide electrical power to the individual loads in the event of a utility power failure. During interruption of utility power, the generator supplies power to the load center, which the load center distributes to the selected or designated circuits of the building. Before activating the generator power supply, the main switch must be disconnected or turned “OFF” to prevent the generator power from back-feeding through the utility conductors. A user must manually configure the switches of the load center to electrically connect the generator power with the series of loads and electrically isolate the utility power from the generator power, and vice-versa. In order to maintain electrical isolation between the generator power input and the utility power input, the connection/disconnection of the utility power supply and generator power supply must be performed in a specific sequence to ensure electrical isolation of the respective input powers. An interlock system has been developed for carrying out this function, and is shown and described in Flegel U.S. Pat. No. 6,621,689 issued Sep. 16, 2003, the disclosure of which is hereby incorporated by reference. While the system shown in the '689 patent controls operation of a main power supply ON/OFF switch and an auxiliary power supply ON/OFF switch, it contains no provisions for controlling operation of neutral switches associated with the main and auxiliary power supplies.
When a bonded neutral generator is connected to the wiring system of a building, the grounding conductor and the neutral conductor are connected in two places within the system. This allows for the return of current back to the generator to flow on both the grounding conductor and on the neutral conductor. Having normal current flow on the grounding conductor limits the ability of the grounding conductor to perform its safety function, and is therefore undesirable. Switching the neutral conductor(s) when switching the power supply conductors prevents any neutral current from flowing on the grounding conductor. Furthermore, it is important to control the sequence of operation of the generator and utility neutral switches when switching between power sources, since supplying power to a device without a neutral may result in failure of the device due to an unpredictable voltage being supplied to the device. To prevent this occurrence, the switching sequence is as follows when switching from main utility power to generator power:                1. Turn off main power;        2. Turn off main neutral;        3. Turn on generator neutral;        4. Turn on generator power.This sequence is reversed when switching from generator power to utility power.        
For the above reasons, it is desirable to provide an input interlock assembly that ensures electrical isolation of the utility power and the generator power during a transfer of the input power from one source to another, and which controls the sequence of operation of neutral switches associated with the utility and generator power supplies.